Device-to-Device (D2D) communications as an underlay to a cellular network have been proposed as a means to take advantage of the proximity of communicating devices and at the same time to allow devices to operate in a controlled interference environment. Typically, such D2D communication shares radio resources with the cellular network. As an example, some radio resources of the cellular network's uplink resources may be reserved for D2D communication. Allocating dedicated radio resources for D2D communication is a less likely alternative as radio resources in general are scarce and dynamic sharing between services utilising D2D communication and services utilising cellular communication is more flexible. In order to achieve the above mentioned controlled interference environment, a solution is to make User Equipments (UEs), involved in the D2D communication, aware of uplink (UL) subframe timing of the cellular network. The cellular network is then configured to schedule the D2D communication to occur in the UL radio resources of the cellular network. This kind of scheduling of the D2D communication is referred to as network assisted scheduling of D2D communication in this disclosure.
D2D communication leads to increased utilization of the radio resources and an intelligent spatial reuse of radio resources is required in order to offload the cellular network and to improve capacity and/or Quality of Service (QoS) levels of the cellular network. Transmissions of the D2D communication typically have lower power than transmissions of cellular communication. The D2D communication may be said to be a direct communication between devices in comparison to cellular communication since transmissions of the D2D communication do not pass via a base station as with the cellular communication between devices. It shall here be said that two UEs in D2D communication with each other are referred to as a D2D pair.
However, transmissions conveyed to cellular and D2D UEs on a same frequency resource are coupled by co-channel interference. The co-channel interference can be adequately estimated in a cell by a network-assisted scheduling policy for D2D communications.
In this context, several studies for finding an efficient D2D network underlying the cellular network as a means to improve utilization of radio resources with a minimal impact on cellular communications have been made. In some distance-based studies in the literature, it has been shown that efficient sharing of the radio resources depends on the distance between D2D users and on their positions with respect to cellular UEs. D2D users refers to two UEs configured for D2D communication. A cellular UE refers here to a UE that is configured for cellular communication via a radio base station. The following studies may be mentioned as examples: K. Doppler, R. Mika, C. Wijting, C. Ribeiro, and K. Hugl, “Device-to-Device Communication as an Underlay to LTE-Advanced Networks”, Institute of Electrical and Electronics Engineers (IEEE) Communications Magazine, vol. 47, no. 12, pp. 42-49, December 2009, G. Fodor, E. Dahlman, G. Mildh, S. Parkvall, N. Reider, G. Mikl´o, and Z. Tur´anyi, “Design Aspects of Network Assisted Device-to-Device Communications,” IEEE Communications Magazine, vol. 50, no. 3, pp. 170-177, March 2012, and M. Belleschi, G. Fodor, and A. Abrardo, “Performance Analysis of a Distributed Resource Allocation Scheme for D2D Communications,” in IEEE Workshop on Machine-to-Machine Communications, Houston, EUA, December 2011.
In the second of the above mentioned studies, it has been shown that the best overall capacity depends mainly on the position of a D2D receiver, e.g. a UE receiving a D2D communication, relative to a cellular UE when reusing Downlink (DL) resources, and similarly relative to the Evolved Node B (eNB) when reusing Uplink (UL) resources.
Cellular communications occurring near an eNB and D2D communications occurring near the cell-edge provides the most favorable scenario for D2D communications according to another study, namely in R. L. Batista, C. F. M. e Silva, J. M. B. da Silva Jr., T. F. Maciel, and F. R. P. Cavalcanti, “Network-Assisted Device-to-Device Communications,” GTEL-UFC-Ericsson UFC.33, Tech. Rep., August 2013, Second Technical Report.
In “Device-to-Device Communication as an Underlay to LTE-Advanced Networks” as mentioned above, an analysis demonstrates the feasibility of the coexistence of both communication types and shows that D2D communications bring benefits in interference-limited local area scenarios. Thus, the potential benefits of D2D communications are strongly constrained by the network topology.
While some of the above mentioned studies have pointed out that the overall capacity of the cellular network supporting D2D communications always outperforms the conventional cellular network, when cellular spectrum resources are reused by D2D communications in so called favorable scenarios, other studies have proposed solutions to extend the range of situations for which D2D is useful through Radio Resource Management (RRM) schemes as mode selection, resource assignment and power allocation. Reference is made to e.g. M. Zulhasnine, C. Huang, and A. Srinivasan, “Efficient resource allocation for device-to-device communication underlaying LTE network”, in Wireless and Mobile Computing, Networking and Communications (WiMob), 2010 IEEE 6th International Conference on, 2010, pp. 368-375, and F. Wang, L. Song, Z. Han, Q. Zhao, and X. Wang, “Joint Scheduling and Resource Allocation for Device-to-Device underlay Communication”, in Proceedings of the IEEE Wireless Communications and Networking Conference (WCNC), 2013, pp. 134-139.
Therefore, RRM techniques applying efficient interference coordination becomes a major issue in cellular networks supporting D2D communications.
In general, most of the proposed schemes for resource assignment for D2D communications considered a pre-selected cellular link such that the scheduling in the cellular network runs independently of establishment of D2D connections.
In “Joint Scheduling and Resource Allocation for Device-to-Device underlay Communication” mentioned directly above, the available channels are firstly allocated to cellular UEs while the D2D pairs form a priority queue for each channel. Then, the eNB sequentially selects the D2D pair which is the first D2D pair in the priority queue and sets the transmit power for each frequency resource. It means that the allocation of channels to cellular UEs is assumed to be fixed while channels are allocated among D2D pairs according to the priority queue. Disadvantageously, multiuser diversity may not be efficiently exploited.
Indeed, it is not possible to ensure that a group including a primary cellular UE and one or more secondary D2D pairs is the most spatially compatible group in each frequency-time resource through a “greedy” approach. Perhaps, all available D2D pairs are spatially incompatible with that primary cellular UE, that is, the primary UE is very close to all available D2D pairs, while a different cellular UE could have better conditions to share the same resource and enable a D2D communication which would provide an increased total throughput. In fact, the choice of a former cellular UE to be the head of a greedy search limits the exploitation of the overall multiuser diversity into a cell, what might also limit the maximum possible throughput gains.
As prior works have focused on finding the most spatial compatible D2D pairs with respect to a primary cellular UE using a greedy approach, they have neglected the total benefits of the multiuser diversity while it is well known that the system capacity can be improved by accurately exploiting such diversity. Reference is made to M. Belleschi, G. Fodor, and A. Abrardo, “Performance Analysis of a Distributed Resource Allocation Scheme for D2D Communications”, in IEEE Workshop on Machine-to-Machine Communications, Houston, EUA, December 2011.
The joint scheduling of cellular and D2D users for shared resources appears in this context as a promising RRM technique that allows high gains with D2D communications and improves the system capacity. However, for scenarios with many cellular UEs and D2D pairs there will be many possible assignments of D2D pairs to each cellular UE as well as many combinations with respect to the number of D2D pairs that could be jointly assigned with each cellular UE and, therefore, the general problem of scheduling a variable number of cellular UEs and D2D pairs in a multi-cell scenario becomes a hard-to-solve optimization problem.
Again in “Performance Analysis of a Distributed Resource Allocation Scheme for D2D Communications”, a heuristic approach achieves performance close to the optimal in terms of spectral efficiency and user fairness through a joint mode selection, resource assignment and power allocation scheme by exploiting multiuser diversity. However, this framework makes an exhaustive search testing all mode selection and resource assignment decisions. Thus, a disadvantage is that a lot of processing is required.